1. Field of the Invention
The present invention relates generally to a cylindrical fluid-filled elastic bushing adapted to isolating or damping vibrations, primarily the vibrations applied thereto in a radial direction thereof, and more particularly to such a fluid-filled elastic bushing capable of effectively damping low-frequency vibrations and effectively isolating medium- and high-frequency vibrations.
2. Discussion of the Prior Art
There is known a generally cylindrical fluid-filled elastic bushing for elastically connecting two members of a vibration system, such that vibrations applied to the bushing in its radial direction are particularly effectively damped or isolated. Examples of such an elastic bushing include suspension bushings for automotive vehicles, and cylindrical engine mounts for mounting an engine on the body of front-engine front-drive vehicles.
For instance, the assignee of the present application developed a fluid-filled elastic bushing as disclosed in U.S. Pat. Nos. 4,749,173 and 4,749,174, which includes (a) an inner sleeve, (b) an outer sleeve disposed radially outwardly of the inner sleeve, (c) an elastic member formed between and elastically connecting the inner and outer sleeves, so as to provide an assembly having a plurality of fluid chambers which are spaced apart from each other in a circumferential direction of the sleeves and are filled with a non-compressible fluid, the assembly further having orifice means for fluid communication between the fluid chambers, and (d) a wing member disposed in at least one of the plurality of fluid chambers to which a vibrational load is applied. The wing member is supported by one of the inner and outer sleeves and the elastic member, and cooperates with the assembly to define a restricted portion within each of the above-indicated at least one fluid chamber (hereinafter referred to as pressure-receiving chamber).
In the fluid-filled elastic bushing constructed as described above, low-frequency vibrations applied to the inner and outer sleeves are excellently damped based on a resistance of the orifice means to flows of the fluid therethrough or on resonance of masses of the fluid in the fluid chambers when the fluid is forced to flow between the fluid chambers through the orifice means, due to changes in the fluid pressures in the fluid chambers, which arise from changes in the volumes of the fluid chambers upon elastic deformation of the elastic member due to displacement of the inner and outer sleeves relative to each other. Upon application of medium- and high-frequency vibrations, on the other hand, the orifice means is placed in a substantially closed state, and the vibrations are effectively isolated based on a resistance of the restricted portion of the pressure-receiving chamber to the fluid flows therethrough or on resonance of the fluid masses in two sections of the pressure-receiving chamber which communicate with each other through the restricted portion. Namely, the restricted portion of the pressure-receiving chamber contributes to lowering the spring constant or vibration transmissibility of the bushing as a whole when the bushing receives the medium- or high-frequency vibrations.
However, the axial end portions of the elastic member which define opposite ends of the pressure-receiving chamber as viewed in the axial direction of the bushing tend to be easily elastically deformed in the axial direction upon application of vibrations to the assembly, since it is generally difficult to construct the assembly such that the axial end portions of the elastic member have a sufficiently large wall thickness.
Usually, the fluid-filled elastic bushing indicated above is installed such that the pressure-receiving chamber formed in the elastic member between the inner and outer sleeves extends in the direction perpendicular to the load-receiving direction in which the vibrational load is applied. In other words, where the elastic bushing is used as a vehicle engine mount, the bushing is installed such that the weight of the engine acts on the bushing as a static or initial load, in the load-receiving direction. This static load causes a certain amount of elastic deformation of the axial end portions of the elastic member, even when no vibrational or dynamic load is applied to the bushing.
The elastic deformation of the axial end portions of the elastic member which define the opposite ends of the pressure-receiving chamber indicated above results in a change in the size of the restricted portion of the pressure-receiving chamber, which is partially defined by the wing member disposed therein. The restricted portion is tuned to a given range of frequency of the vibrations desired to be damped. That is, the size and configuration of the restricted portion are adjusted to effectively isolate a medium- and high-frequency range of vibrations. However, the deformation of the axial end portions of the elastic member causes a change in the tuned frequency range of the vibrations, resulting in reduced capability of the elastic bushing to isolate the medium- and high-frequency vibrations.